1. Field of Invention
This invention relates to engagement devices for amphibious surface vehicles, particularly for such devices to be operated in tractive and/or propulsive modes over varying terrain attributes, surface conditions, and though substances exhibiting high fluidity characteristics.
2. Prior Art
The current art of surface transport vehicles utilized numerous types of engagement devices such as, wheels, endless tracks, steppers, articulating members, and rails. Such transport devices perform adequately within their respective surface domains, however struggle and become cumbersome when transitioning to other surfaces beyond their intended operational envelopes.
For example, this is typical of conventionally wheeled vehicles transitioning from a developed road bed to dry sand, loose gravel, or moisture saturated soils resulting in the shear failure of the surface, thus penetrating therein. This usually results in the loss of forward momentum and available traction, thus becoming completely immobilized within the surface encountered. A method to alleviate this problem as found in the construction, mining, agriculture and timber industries which commonly utilize large wheel diameters and widths to increase the contact area or footprint to aforesaid unimproved surfaces. This approach reduces the ground pressure exerted therefore lowering the shearing forces imposed to the underlying terrain.
The efficient mobility of a wheeled surface vehicle is dependant on several factors: slope of the surface (grade), internal wheel friction, contact friction (grip) and rolling resistance. The latter is directly related to the amount of deformation of the wheel and the load bearing surface when in contact, thus creating this additional resistance. Rolling resistance is analogous to ascending a constant positive slope and when this slope is combined to the actual grade, it can overcome the provided traction (grip), thus spinning occurs. Also, rolling resistance requires additional power and torque to overcome due to continuously traversing this added slope thus more fuel consumption and the loss of available pulling force. To reduce rolling resistance by distributing the load to a greater contact area thus increasing traction and decreasing penetration into the ground surface results in greater efficiency and effective pulling power. This is the rationale in the industries mention above, but a scalability limit is soon reached with very large diameter wheels, by sacrificing torque or rotational leverage (rim-pull), thus insufficient power to pull or haul a payload over yielding surface conditions.
Another terrain engagement device that brings the surface along with it, such as a track laying vehicle, which nearly negates rolling resistance by providing a large contact area, thus limiting penetration. Also, track laying vehicles are very agile in steep terrain more so than conventionally wheeled devices by generating large amounts of traction and leverage or drawbar pull which is analogous to wheel rimpull mentioned above. They can negotiate low ground pressure areas due to low downward forces exerted by employing wide tracks thus reduce yielding effects of the underlying surface. A critical tradeoff occurs though, with tracked vehicles in performance in speed, therefore not an effective conveyance on improved, hard or paved surfaces where higher velocities can be attained by wheeled vehicles. Albeit, tracks are very robust, they have other major drawbacks such as a very short service life, a high maintenance schedule, continual part replacement, and prohibitive energy consumption. Also, a multi-linked track or chain is as strong as its weakest link and this is the ‘Achilles Heel’ of track laying or endless track vehicles were redundancy is paramount, such as with military, search rescue, and remote operations.
However, the vehicle performance envelope can be expanded by combining various terrain engagement systems in complimentary configurations. This usually is impractical and creates unneeded complexity and expenses with the same inherent disadvantages mentioned above. Several prior art vehicles utilize such methods and have only found limited success. Also, amphibious vehicles may utilize auxiliary propulsion devices when waterborne. These propulsion devices range from screw propellers, water jets, paddle wheels, or ducted fans such as with hovercraft. By just utilizing a single propulsive drive device to do both surface engagement and to impel thrust, would greatly simplify the operation and cost of the vehicle.
The following prior art will described several of the numerous innovations to overcome some of the disadvantages mentioned above
Harvey, in U.S. Pat. No. 5,881,831 teaches a multi-terrain amphibious vehicle adapted for travel across various types and attributes. The vehicle includes a chassis assembly which extends in a longitudinal direction; a plurality of propulsion members rotatably coupled to the chassis assembly for propelling the vehicle across a given surface; and, a control mechanism for controlling the rotational velocities and phases of the propulsion members. Each propulsion member essentially resembles a mutilated circular wheel where the mutilated portion of perimeter segment is used to engage or ‘pushes off’ of the underlying surface. However, the propulsion members require a complicated control mechanism to collectively cooperate so as to operate effectively over various terrains. Also, the use of a circular perimeter segment for the propulsion members creates the same disadvantages aforementioned for wheeled vehicles.
Reid, in U.S. Pat. No. 4,102,423 shows a ground traction device which is non-circular in its periphery and each member containing a three lobed tire, preferably constructed of rubber. The periphery containing three individual arcs arranged in the form of an equilateral triangle. Members may be situated adjacent or axially spaced apart and have peripheries of any other suitable shape, such as two or four sided. It is intended to operate and tramp over soft ground and when transitioning to a hard surface, the ground engaging member behaves as a circular wheel of constant radius by compressing the rubber tire portion. However, this adaptation could create excessive amount of heat buildup in the rubber tire due to constant compression and rebound cycle when operated on a hard surface. The rubber or other flexible material under these conditions would eventually fail and de-vulcanize or delaminate, thus rendering a vehicle equipped with ground traction devices inoperative. Also, pressure sensitive soft terrain would be adversely affected by the penetrating ‘digging’ lobes when not compressed by the underlying surface.
Sfredda, in U.S. Pat. No. 2,786,540 illustrates a non-circular wheeled vehicle with similar phasing of the ground contacting wheels as with Harvey's patent where a set is “out of phase”. This relationship contributes to good traction, so as to permit differently shaped edge portions of different wheels to simultaneously contact the ground at all times. This achieved by vertically reciprocating the axis of rotation within a slot so as to limit its travel, and to permit smooth contact with a horizontal plane. Other, non-circular, multi-sided configurations (polygons) as a hexagon, octagon or the like may be employed. Sfredda teaches the use of a roller and a cam disk to urge or float the axle within a limiting slot as it rotates by a driven geared pinion. However, two or three sided (lobed) configurations seem excluded due to gear interference or impracticality with a cam system. Also, the device contemplated is limited to a pair of non-circular wheels per wheel site. This would also cause ‘digging’ within pressure sensitive terrain since each corner edge portion would contact the surface simultaneously.